Layers are known which strongly absorb sunlight and have low emissivity in the infrared region, as are layers which reflect sunlight and which emit in the infrared region. Such layers are on the one hand used in collectors of the sun's energy as so-called "super Black" layers or in devices rejecting the sun's energy as so-called "super white" layers.
It is known to use such layers for heating water for heating purposes. The heat which can be gained from the sunlight is given by the following equation: EQU N = S -- R -- I,
where N is the useful output, S the incident energy radiation/m.sup.2 per second based on the solar constants, R the reflection/m.sup.2 per second and I the reflection in the infrared region/m.sup.2 per second.
The figure for R decreases as the absorption coefficient of the layer for the sunlight increases. The figure for I on the other hand, increases proportionally to the absorption coefficient in the infrared region and as T.sup.4, where T is the absolute temperature.
From this it follows that water storage apparatus provided with a physically black surface layer, has a low efficiency, since although R is very small, I rapidly increases as the water temperature increases and reaches the value of S in the region of the boiling point of the water. At this temperature therefore the incident radiation and the longwave reflection are in equilibrium. In the case of water storage apparatus for domestic purposes the desired temperature is, as a rule, only 40 or 50.degree. . At these low temperatures, however, only small quantities of energy can be stored for any given volume of the storage tank, since the storage capacity of the water increases proportionally to the temperature. This is the reason why where water is used as the heat storage medium, very poor efficiencies have to be expected.